1. Field of the Invention
The present invention relates to a novel aluminum nitride sintered body having a high purity and a low volume resistivity, and metal embedded articles, electronic functional materials and electrostatic chucks utilizing this aluminum nitride sintered body.
2. Description of Related Art
At present, in film forming processes comprising the steps of transferring, exposing to light, chemical vapor depositing, sputtering and the like, and in the succeeding steps of micromachining, washing, etching, dicing and the like, of a semiconductor wafer, electrostatic chucks are used for attracting and holding the semiconductor wafer. As a substrate of such electrostatic chucks, dense ceramics have been drawing attention. Particularly, in apparatuses for manufacturing semiconductors, halogenous corrosive gases such as ClF.sub.3 and the like are frequently used as etching gases or cleaning gases. Moreover, in order that the semiconductor wafer is rapidly heated or cooled while being chucked, the substrate of the electrostatic chuck is desired to have a high thermal conductivity. It is further desired to have such a high thermal shock resistance that it may not fracture due to a rapid temperature change. A dense aluminum nitride has a high corrosive resistance against halogenous corrosive gases as mentioned above. Moreover, it has been known that such an aluminum nitride is a high thermal conductive material and its volume resistivity is not less than 10.sup.14 .OMEGA..multidot.cm at room temperature. Further, it is also known that aluminum nitride has a high thermal shock resistance. Therefore, it is accounted preferable to form a substrate of an electrostatic chuck for semiconductor manufacturing apparatuses of an aluminum nitride sintered body.
On the other hand, in the semiconductor-manufacturing apparatuses, in order to use an electrostatic chuck as a susceptor for holding a semiconductor wafer, it is necessary to enhance the attracting force of the electrostatic chuck, and consequently it is necessary to decrease the specific resistance of the substrate. For example, in Japanese Patent Publication No. 7-19831, in order to increase the attracting force of the electrostatic chuck by decreasing the resistance of the insulating dielectric layer of the electrostatic chuck, an insulating material having a high volume resistivity is incorporated with a conductive or semiconductive material to adjust the resistivity of the insulating dielectric layer to not more than 10.sup.13 .OMEGA..multidot.cm. Moreover, in Japanese Patent Laid-Open Publication No. 2-22166, ceramics starting materials containing alumina as a main component are fired in a reducing atmosphere to produce dielectric ceramics for electrostatic chucks. In this case, the ceramic starting material is made to contain an alkali earth metal and a transition metal in amounts as oxide of 1-6% and 0.5-6%, by weight, respectively. In this method, for example, it is intended to increase the dielectric constant, concurrently decreasing the volume resistivity to 10.sup.12 -10.sup.18 .OMEGA..multidot.cm, by incorporating TiO.sub.2 with the alumina ceramics, thereby to obtain a high attracting power.
However, according to such a method, there will be posed problems such that products produced by corrosion of alkali earth metals and transition metals yield particles.
However, highly purified aluminum nitride sintered bodies are not suited for forming a substrate of electrostatic chucks for semiconductor-manufacturing apparatuses, because of such a high volume resistivity thereof as at least 10.sup.14 .OMEGA..multidot.cm. In order to provide them with a sufficient attracting force, it is necessary to form an extremely thin insulating dielectric layer, 300 .mu.m thick or less. However, it has been found that when such a thin insulating dielectric layer contacts with halogenous corrosive gas or plasma, during its long time use, there is a possibility of causing an insulation breakdown and the like occurring from a starting point on any of reactant layers on the surface of the insulating dielectric layer. From this point of view, it has been found that the insulating dielectric layer is preferred to be 500 .mu.m thick or more.
However, in conventional electrostatic chucks made of an aluminum nitride sintered body, such a thick insulating dielectric layer results in decrease of the attracting force of the electrostatic chucks. Particularly in a low temperature region where a volume resistivity is high, it has been difficult to provide a sufficient attracting force. Particularly, it is at a low temperature of from -50.degree. C. to -60.degree. C. that dry etching processes are conducted, and it is at relatively a low temperature of around 100.degree. C. that highly densified plasma CVD processes are conducted. Therefore, in such low temperature processes, it has been difficult to constantly provide a predetermined attracting force.
For this reason, the present inventors have restudied the aluminum nitride sintered bodies per se.
For example, in an electrostatic chuck comprising an aluminum nitride substrate, the effect of adding a low resistivity material to the aluminum nitride substrate was studied according to the description in Japanese Patent Publication No. 7-19831. By this method, the volume resistivity of the aluminum nitride sintered body was able to be reduced to not more than 10.sup.13 .OMEGA..multidot.cm. However, with this electrostatic chuck, there is a possibility of causing pollution to semiconductor by separation of the added low resistant metal or the like from the surface of the substrate.
Alternatively, it has been proposed to improve a thermal conductivity as well as a density of aluminum nitride by adding an oxide or carbonate of a rare earth element such as yttrium or the like, as a sintering assistant, to an aluminum nitride starting material (Japanese Patent Publication No. 63-46032). Using such a sintering assistant, dense aluminum nitride sintered bodies can be manufactured even by a normal pressure sintering method. However, such aluminum nitride sintered bodies have a high volume resistivity, and even those of about 99% relative density have a volume resistivity on a level of 10.sup.13 -10.sup.15 .OMEGA..multidot.cm.